1. Field of the Invention
The subject matter of the invention concerns an arrangement and method, especially using the arrangement of the invention, for analyzing exhaust gas from an internal combustion engine, in which at least a part of the exhaust gas flows past a .lambda.-sensor and through an IR spectrometer after passing through a condenser connected to the IR spectrometer upstream of the IR spectrometer in an exhaust gas flow direction.
2. Prior Art
The so-called .lambda. value is an important parameter in relation to fuel combustion in internal combustion engines and the ever more stringent requirements for reduction of exhaust gas emission from this type of motor. The .lambda. value is equal to the actual air/fuel ratio divided by the stoichiometric air/fuel ratio. An engine produces less noxious exhaust gases, the closer the .lambda. value is to one. Methods to determine or control the .lambda. value are thus important. Exhaust gas parameters are consulted for determination and/or control purposes.
In the book "Automotive Electronics Handbook", McGraw Hill, Inc., 1995, in Chapter 6, "Exhaust Gas Sensors", sensors are described, which include a sensor (designated in the following as a .lambda.-sensor), which is operated as a combination of a sensor based on the Nernst principle and a diffusion flow probe, which are immersed in the flow of exhaust gas from the internal combustion engine. This .lambda.-sensor is used for control of the .lambda. value of the .lambda.-value-dependent measured flow. A
In the paper "Air-Fuel Ratio Sensor for Rich, Stoichiometric and Lean Ranges" of S. Suzuki, among others, published in the SAE Technical Papers Series 860408, pp. 18ff. (reprinted from SP-655 Sensors and Actuators (1986)) a .lambda.-sensor is described, with which it can be established whether the .lambda. value is in the rich, stoichiometric or lean range.
Determination of the .lambda. value from the signal of a .lambda.-sensor is known. The water gas equilibrium is included in the calculation. Since hydrogen and the water vapor concentrations are not measured in the gas phase, the water gas equilibrium is called upon for determination of the .lambda.value. A value of 3.6 is used for the equilibrium constant K.sub.p, but only immediately downstream of the engine. An exhaust gas catalytic converter can greatly change the water gas equilibrium. Current catalytic converters scarcely change the water gas equilibrium of course, but only because sulfur from the fuel blocks these changes. If the fuel does not contain sulfur, the water gas equilibrium changes greatly in the catalytic converter. Thus in this latter case the .lambda. value can only be determined immediately behind or downstream of the engine.
One other method of calculating the .lambda. value is described in the paper of J. Brettschneider, "Calculation of the air ratio of air-fuel mixtures and the influence of measurement errors on .lambda.", in Bosch Technische Berichte (Technical Report), Bd. (Vol) 6, Heft(part) 4 (1979), pp. 177 to 186 and Bosch Technische Berichte (Technical Report), Heft(part) 56 (1994), pp. 30 to 45. In this method CO, CO.sub.2 and HC (hydrocarbons in the exhaust gas) are determined by means of an IR spectrometer, for example O.sub.2 polarographically or by EPR spectroscopic methods, and, if necessary, No by means of chemiluminescence measurements, since water vapor would be condensed prior to that measurement. When the determination of NO must be avoided because of its expense, some accuracy is lost. Rich exhaust gas and lean exhaust gas having .lambda. values approaching 1 (in the latter case having .lambda. values that lie just above one) contain hydrogen. The water gas equilibrium is included in the calculation. Thus the above-mentioned problems also occur here in the Brettschneider method.
An additional method for calculation of .lambda. has been given by W. Simons, who has determined that an oxygen measurement provides an additional degree of freedom, which can be used in order to calculate the equilibrium constant K.sub.p (see the article "Calculations for determination of the excess air coefficient" in MTZ Motortechnische Zeitschrift (Motor Engineering Journal) 46, 7/8 (1985), pp. 257 to 259). This method thus does not have 'the inherent limitations of the Brettschneider method in regard to the place at which the exhaust gas sample is taken, but is not very accurate.
Finally a method of calculation of .lambda. is described in an article entitled "An Algorithm for Calculating the Air/Fuel Ratio from Exhaust Emissions", by W. M. Silvis, Nr. 970514, Society of Automotive Engineers (1997), pp. 141 to 152. In the described algorithm the water moles are determined either according to the Brettschneider method, in which a value for the equilibrium constant K.sub.p is between 3.5 and 3.8, or according to the Simons method by means of the nitrogen and the mole balance.